Deniz üzeri alçak irtifa askı uçuşlarında helikopterde oluşan tuzlu su spreyi kirlenmesinin sayısal incelenmesi

Numerical investigation of saltwater spray contamination on helicopter during low-altitude maritime hover flights

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Tez No: 1022194
Yazar: UFUK ŞAHİN
Danışmanlar: DOÇ. DR. SERTAÇ ÇADIRCI
Tez Türü: Yüksek Lisans
Konular: Makine Mühendisliği, Mechanical Engineering
Anahtar Kelimeler: Belirtilmemiş.
Yıl: 2026
Dil: Türkçe
Üniversite: İstanbul Teknik Üniversitesi
Enstitü: Lisansüstü Eğitim Enstitüsü
Ana Bilim Dalı: Makine Mühendisliği Ana Bilim Dalı
Bilim Dalı: Isı-Akışkan Bilim Dalı
Sayfa Sayısı: Belirtilmemiş.

Özet

Helikopterler çeşitli manevra kabiliyetleri nedeniyle kara ve deniz operasyonlarında sıklıkla kullanılan gelişmiş hava araçlarıdır. Arama kurtarma, dikey ikmal ve yangın söndürme gibi görevler deniz üstü koşullarda uçma gerekliliği getirmektedir. Bu koşullarda gerçekleşen uçuşlar çeşitli çevresel ve uçuş performansı zorlukları yaratmaktadır. Özellikle uçuş irtifasının azaldığı durumlarda helikopter rotorunun aşağı yönlü yüksek hızlarda bir akış alanı yaratması (downwash), su yüzeyindeki kararsız deformasyonun da etkisiyle uçuş stabilitesini olumsuz etkilemesinin yanında su yüzeyinden kopan taneciklerin oluşturduğu sprey bulutunun helikopteri kirletmesi operasyonel risklere neden olmaktadır. Deniz suyundan oluşan sprey bulutunun helikopter gövdesinde birikerek bir sıvı filmi oluşturması ve helikopterin motor alıkları veya filtre girişleri gibi açıklıklardan girerek çeşitli iç aksamlara nüfuz etmesi bu kirliliğin tanımıdır. Deniz suyuyla taşınan tuzun su buharlaştıktan sonra sprayin ulaştığı bölgelerde tortu şeklinde birikmesi malzeme korozyonu tehlikesini beraberinde getirmektedir. Özellikle motor bölmesinde oluşan kirlenmeler ciddi performans kayıplarına ve gerekli yıkamaların yapılma zorunluluğunu beraberinde getirmektedir. Literatürdeki çalışmalar helikopterlerin deniz üstü alçak irtifa uçuşlarında rotor kaynaklı oluşan bulutun oluşturduğu kirlenmeleri ve etkilerini çeşitli testlerle ortaya koymuşlardır. Bunun yanında Hesaplamalı Akışkanlar Dinamiği (HAD) ile sayısal modelleme yaklaşımını kullanan çalışmalar sadece küçük ölçekli rotorlar için aerodinamik etkilerin incelenmesi ile sınırlı kalmıştır. Bu çalışmalarda, su yüzeyine yakın konumlandırılan rotorların oluşturduğu momentumun durgun yüzeyi parçalayarak sıçrama oluşturması gözlemlenmiş olup yüzeyler üzerinde birikim ve dağılımları gibi çıktılar bulunmamaktadır. Bu karmaşık fizik zamana bağlı olarak Reynolds Ortalamalı Navier-Stokes (RANS) denklemleri kullanılarak modellenmiştir. Hacimsel Akışkan (VOF) yaklaşımı ile iki fazlı akışkanın etkileşimi modellenmiş olup, helikopter yüzeyinde oluşan birikimin ele alınması için Akışkan Filmi modeli kullanılmıştır. Helikopterin ana ve kuyruk rotorları, Star CCM+ ortamında Blade Element temelli Sanal Disk (Virtual Disk) yaklaşımıyla modellenmiştir. Gerçekleştirilen simülasyonlar 10, 20 ve 30 ft irtifalarında hover koşulları ile rüzgarlı ve rüzgarsız senaryolar için yürütülmüştür. Elde edilen sonuçlar, irtifanın kirliliğin şiddeti üzerindeki belirleyici etkisini ortaya koymuştur. En kritik irtifa olarak 10 ft belirlenmiş olup bu irtifada gövde üzerinde en yoğun sıvı filmi birikimi gözlemlenmiş, motor alıkları ve filtre girişlerinden ölçülen su kütlesel debi değerleri diğer irtifalara kıyasla belirgin şekilde yüksek çıkmıştır. Açıklıklardan içeri giren su fazının kütlesel debisi, her bir motor alığı ve filtre girişi için simülasyon süresi boyunca zamana bağlı olarak ayrı ayrı izlenmiş; raporlanan değerler, çözümün son saniyelerinde elde edilen ortalama debilere karşılık gelmektedir. Sağ motor alığına giren ortalama su debisi yaklaşık 0,42 g/s, sol motor alığına giren ise yaklaşık 0,14 g/s olarak hesaplanmıştır. Aynı irtifada arka filtre girişlerinde sol tarafta ortalama 0.12 g/s, sağ tarafta ise 0.022 g/s debi ölçülmüş; ön filtre girişlerinin her iki tarafında ise birbirine yakın ve yaklaşık 0.09 g/s düzeyinde değerler elde edilmiştir. Bu asimetrik dağılımın önden gelen rüzgar ile rotor dönüş yönünün birleşik etkisinden kaynaklandığı değerlendirilmektedir. İrtifa 20 ft'ye çıktığında kütlesel debi değerleri 10⁻⁵–10⁻⁷ g/s mertebesine gerilemiş, 30 ft'de ise gövde üzerindeki film dağılımı ve su fazı debileri ihmal edilebilir düzeyde kalmıştır. Rüzgar koşullarının etkisi incelendiğinde, önden gelen rüzgarın sprey bulutu morfolojisini ve gövde üzerindeki kirlenme bölgelerinin dağılımını değiştirdiği, ancak toplam kirlenme miktarları üzerindeki etkisinin sınırlı kaldığı görülmüştür. Motor alıkları ve filtre girişlerinden okunan kütlesel debiler giriş yüzey alanlarına göre normalize edildiğinde, küçük açıklıklara sahip filtre girişlerinde birim alan başına düşen su fazı miktarının motor alıklarına kıyasla belirgin şekilde yüksek olduğu gözlemlenmiştir. Yüzeyden koparak taşınan tuzlu su damlacıkları ile taşınan tuz miktarının zamanla birikiminin tahmin edilebilmesi amacıyla deniz suyunun tuzluluk oranı sabit %3,5 olarak kabul edilmiştir. Tuzlu suyun ulaştığı bölgelerde su fazının tamamen buharlaştığı ve geriye yalnızca tuz kristallerinin kaldığı varsayımı esas alınarak yapılan hesaplamalar sonucunda, bir saatlik askı uçuşu süresince sağ motor alığında yaklaşık 53 g, sol motor alığında ise yaklaşık 18 g tuz birikeceği öngörülmüştür. Elde edilen bu değerler, korozyon oluşumu ve motor performansındaki olası kayıpların değerlendirilmesinde referans niteliği taşımaktadır.

Özet (Çeviri)

Helicopters are advanced rotary-wing aircraft that are widely employed in both land and maritime operations due to their exceptional maneuverability and vertical take-off and landing (VTOL) capability. Unlike fixed-wing aircraft, their ability to fly at low speeds and sustain hover makes them uniquely suited for missions such as search and rescue, vertical replenishment, and maritime firefighting including water scooping operations. In these flight regimes, where the altitude above the surface is greatly reduced, the aerodynamic behavior of the helicopter cannot be adequately characterized by free-air conditions alone; the interactions between the rotor-induced flow field and the proximate surface must also be taken into account. During low-altitude hover over a water surface, the intense downward flow generated by the main rotor interacts directly with the sea surface. This interaction, commonly referred to as the near water effect, produces unstable surface deformations and ultimately causes water particles to detach from the surface and form a dense spray cloud that envelops the entire airframe. This spray cloud constitutes a contamination hazard: saltwater droplets impinge on the fuselage to form a liquid film, while a portion of the spray is ingested through open sections of the airframe such as engine inlets and filtered avionics cooling passages, allowing saltwater to penetrate critical internal components. As the water evaporates, the dissolved salt accumulates as deposits in the regions reached by the spray, introducing the risk of material corrosion. Contamination within the engine compartment in particular can lead to severe performance losses and necessitate mandatory wash procedures, imposing significant operational and maintenance burdens. Existing studies in the literature have characterized the contamination effects of rotor-induced spray during low-altitude overwater helicopter flight through various experimental test programs. However, numerical modeling studies employing Computational Fluid Dynamics (CFD) have remained limited to investigations of aerodynamic effects for small-scale rotors. While such studies have successfully observed the depression and splashing of a quiescent water surface driven by the rotor momentum, they have not produced outputs such as film accumulation and distribution on helicopter surfaces or quantified ingress through critical openings. The present work addresses this gap by developing a comprehensive CFD framework that captures the full chain of coupled phenomena from rotor downwash through spray generation to airframe contamination. The primary objective of this study is to identify and quantify the regions of a helicopter airframe that are exposed to salt contamination during low-altitude maritime hover operations, by simulating the rotor-induced multiphase flow using a CFD approach. Two key aspects of contamination are investigated: the formation and distribution of a liquid film on the external fuselage surfaces, and the mass flow rates of the water phase entering through critical open sections of the airframe. The engine inlets, through which air is drawn into the propulsion system, and the filtered inlets through which avionics cooling fans draw air, represent the primary ingestion pathways of concern. The dependence of both film accumulation and water ingress on hover altitude is examined under representative wind and no-wind conditions. To achieve these objectives, a generic utility helicopter geometry with representative average dimensions was constructed, and the aerodynamic influence of the main and tail rotors was represented using a Virtual Disk model based on the Blade Element approach. The coupled Volume of Fluid (VOF) and Fluid Film models were employed to simultaneously resolve the free surface interaction and track the accumulation of liquid film on the airframe. Transient simulations were carried out using the Siemens Star-CCM+ commercial solver with a trimmed hexahedral mesh and the k-omega SST turbulence model, and mesh independence was verified prior to the production runs. Hover conditions were simulated at three altitudes of 10, 20, and 30 ft, with a 6-knot headwind applied as the primary wind scenario alongside a calm condition at 10 ft for comparison. The complex multiphase physics governing rotor-induced spray generation and its interaction with the helicopter airframe were modeled using the unsteady Reynolds-Averaged Navier-Stokes (RANS) equations. The two-phase air-water system was treated as an incompressible flow and resolved using the Volume of Fluid (VOF) method, which tracks the interface between the two immiscible phases by solving a transport equation for the volume fraction of each phase alongside the shared momentum equations. This approach enables the free surface deformation driven by rotor downwash to be captured continuously in time, including the formation of surface cavities, the generation of spray droplets through splashing, and the subsequent transport of the water phase toward and onto the airframe. To evaluate the extent of liquid film formation on the helicopter fuselage, the Fluid Film model was activated on the aircraft shell boundaries and integrated with the VOF solver. This model tracks the impingement of spray droplets on the fuselage surfaces and the subsequent accumulation, spreading, and drainage of the resulting thin liquid layer. In addition to the external film, the water mass flow rates entering through the helicopter's critical open regions were monitored throughout each simulation. The engine inlets on the left and right sides of the fuselage, the front avionics filter inlets, and the rear avionics filter inlets were each treated as separate monitoring surfaces, providing time-resolved ingress data for all critical locations. The aerodynamic influence of the main and tail rotors was represented using a Virtual Disk approach based on Blade Element theory. This method imposes the rotor-induced momentum source terms on a disk region within the computational domain without resolving the rotating blade geometry explicitly, making the approach computationally tractable for the transient multiphase simulations required by this study. Turbulence closure was achieved using the k-omega Shear Stress Transport (SST) model, which is well suited to flows with adverse pressure gradients, flow separation, and surface impingement effects characteristic of the near-surface hover environment. The computational domain was discretized using a trimmed hexahedral mesh with local refinements concentrated in the rotor wake region and near the air-water interface. A mesh independence study was conducted using three progressively refined grid configurations: a coarse mesh of approximately 2.6 million cells, a medium mesh of approximately 3.8 million cells, and a fine mesh of approximately 8.6 million cells. The independence of the solution was assessed by comparing velocity profiles and volumetric water fraction distributions along a vertical probe line located in the nose region of the helicopter, directly beneath the main rotor disk. The medium and fine mesh configurations produced results in close agreement with one another, while the coarse mesh exhibited notable deviations. Accordingly, the medium mesh was selected for all production simulations as it provides a suitable balance between numerical accuracy and computational cost. Transient simulations were performed at hover altitudes of 10, 20, and 30 ft under a 6-knot headwind condition, with an additional no-wind simulation at 10 ft. In all cases, the rotor downwash was observed to generate a spray cloud of water droplets that accumulates on the helicopter surfaces or enters its systems. The hover altitude above the sea surface was identified as the dominant parameter governing the intensity of contamination, both in terms of fuselage film accumulation and water ingress through critical inlets. The effect of the headwind on the spray cloud morphology was clearly observed: the 6-knot headwind stretches the cloud rearward, producing an elongated shape in the longitudinal direction rather than the approximately symmetric circular distribution seen under no-wind conditions. An examination of the film distribution on the fuselage reveals that the difference between the windy and no-wind conditions lies primarily in the regional intensity variations of the distribution rather than in the overall contamination quantity. In other words, the headwind modifies where on the fuselage the film accumulates most intensely, but does not substantially alter the total amount of water deposited at a given altitude. When the results are compared across altitudes, the highest film accumulation is observed at 10 ft, and the accumulation decreases significantly as altitude increases to 20 and 30 ft. At 10 ft the liquid film extends extensively across the lower fuselage and tail section, reflecting the intensity of spray impingement at this critical hover altitude. At 30 ft, the film distribution on the fuselage is negligible throughout the analysis period, indicating that the spray cloud generated at this altitude does not effectively reach the airframe. The time-dependent water mass flow rates entering through the engine inlets and avionics filter openings were evaluated over 15-second simulation periods. Consistent with the film accumulation trends, the highest ingress rates occur at 10 ft altitude, with a significant reduction observed at 20 ft and near-zero values at 30 ft. At 10 ft, the average ingress rate computed over the final 5 seconds of simulation at the right engine inlet is approximately 0.42 g/s, while the left engine inlet records approximately 0.14 g/s. This asymmetry, amounting to a factor of approximately three between the two sides, is attributed to the combined effect of the headwind direction and the rotational sense of the main rotor, which together create an asymmetric downwash and spray entrainment pattern around the fuselage. Among the filter inlets, the left rear filter records an average ingress of approximately 0.12 g/s and the right rear filter approximately 0.022 g/s, while the front filter inlets on both sides exhibit nearly equal ingress rates of approximately 0.09 g/s. To place these ingress rates in context, the monitored mass flow rates were normalized by the surface area of each opening to yield a water mass flux in g/m²·s, using areas of 0.788 m² for each engine inlet and 0.0124 m² for each filter inlet. Although the engine and filter inlets were prescribed mass flow boundary conditions of comparable order, the normalized fluxes reveal that the filter inlets, owing to their much smaller geometric area, draw substantially more water per unit area than the engine inlets. At 10 ft, the right engine inlet corresponds to a flux of approximately 0.53 g/m²·s and the left engine inlet to approximately 0.18 g/m²·s, whereas the rear filter inlets reach approximately 9.68 g/m²·s on the left and 1.77 g/m²·s on the right, and the front filter inlets approximately 7.26 g/m²·s on both sides. This indicates that the small filtered openings, despite their modest absolute ingress, experience a locally intensified contamination loading. An examination of the velocity field around the airframe clarifies the origin of this left-right asymmetry. At the right engine inlet, which records the higher ingress, the flow approaches the inlet plane at a more nearly perpendicular angle, whereas at the left inlet the velocity vectors exhibit a more tangential character. Consistently, the regions of greater water ingress coincide with greater local liquid film thickness around the corresponding openings, indicating that ingestion is closely tied to the accumulation of film in the immediate vicinity of each opening. At 20 ft altitude, the mass flow rate values at all monitored locations decrease to the order of 10⁻⁵ to 10⁻⁷ g/s, representing a reduction of several orders of magnitude relative to the 10 ft case. A further decline is observed at 30 ft, where the ingress rates are negligible for the entire duration of the simulation. These results confirm that 10 ft represents the most critical altitude from both a fuselage film accumulation and a water ingestion standpoint, and are consistent with experimental findings in the literature reporting that salt formation occurs at altitudes of 30 ft and below during hover operations. To translate the computed water ingress into an indicator of contamination severity, the salt mass carried by the ingested seawater was estimated from the water mass flow rates assuming a typical seawater salinity of 3.5%. On this basis, a one-hour maritime hover at 10 ft would transport approximately 53 g of salt into the right engine inlet and approximately 18 g into the left, with smaller quantities entering through the filter inlets. Although the absolute salt mass entering the filters is lower than that of the engine inlets, the local salt loading per unit area is more critical at these small openings. These estimates provide quantitative reference points for assessing corrosion risk and engine performance degradation, and they diminish rapidly with altitude, becoming negligible at 30 ft. This study presents a comprehensive numerical investigation of sea spray contamination on a helicopter airframe during low-altitude maritime hover operations, constituting the first CFD study to address these coupled phenomena for a full helicopter geometry. The coupled VOF and Fluid Film modeling framework implemented in Siemens Star-CCM+, together with a Virtual Disk rotor model and the k-omega SST turbulence closure, successfully captured the rotor downwash-free surface interaction, spray generation, fuselage liquid film accumulation, and water ingress through engine inlets and avionics filter openings across a range of operationally relevant hover altitudes and wind conditions. The results establish 10 ft as the most critical hover altitude from a contamination perspective, with the highest fuselage film accumulation and the highest water ingress rates recorded at this altitude for both engine inlets and avionics filters. A pronounced asymmetry between the left and right engine inlets was identified, with the right inlet receiving approximately three times the water ingress of the left, a consequence of the interaction between the headwind direction and the main rotor rotation. Upon increasing altitude to 20 ft, contamination levels drop by several orders of magnitude, and at 30 ft both fuselage film formation and inlet ingress are effectively negligible. The headwind primarily redistributes the spatial pattern of contamination on the fuselage without substantially altering the total contaminated quantity at a given altitude. The numerical framework developed in this work provides a quantitative basis for informing operational altitude limits, guiding maintenance planning, and supporting design improvements for helicopters operating in maritime environments. Future work may extend the present analysis to encompass a wider range of wind speeds and directions, varying sea state conditions, and the integration of salt deposition and corrosion prediction models to further quantify the long-term material degradation associated with sea spray ingestion during low-altitude overwater operations.

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