Investigation of influence of compact actuators on air cavity in water flow under recessed hull

Air cavity drag reduction is one promising method for reducing power consumption of ships. Its current practical applications are rather limited, owing largely to the fact that air cavity size and shape change drastically in response to variations in ship attitude, motions and speed, as well as sea conditions. This study explores how deployment of moveable hydrodynamic actuators near the air cavity on a small-scale simplified hull form can effectively increase the air cavity size in adverse hull positions. Experimentally investigated actuators included an adjustable plate in the front part of an air cavity, a stern spoiler, and a hydrofoil with regulated attack angle and streamwise position beneath the hull. In the cases of significant hull trims that are challenging for maintenance of long air cavities, optimal actuator placement increased cavity length by nearly 110% from its degraded state at negative trim and by 24% at positive trim. Actuator effects were more pronounced at higher water speeds.

Using shock control bumps to improve engine intake performance and operability

Shock control bumps can be used to control and weaken the shock waves that form on engine intakes at high angles of attack. In this paper, it is demonstrated how shock control bumps applied to an engine intake can reduce or eliminate shock-induced separation at high incidence, and also increase the incidence at which critical separation occurs. Three-dimensional Reynolds-average Navier–Stokes (RANS) simulations are used to model the flow through a large civil aircraft engine intake at high incidence. The variation in shock strength and separation with incidence is first studied, along with the flow distribution around the nacelle. An optimisation process is then employed to design shock control bumps that reduce shock strength and separation at a fixed high incidence condition. The bump geometry is allowed to vary in shape, size, streamwise position and circumferential direction around the nacelle. This is shown to be key to the success of the shock control geometry. A further step is then taken, using the optimisation methodology to design bumps that can increase the incidence at which critical separation occurs. It is shown that, by using this approach, the operating range of the engine intake can be increased by at least three degrees.

Experimental Study on the Influence of the Streamwise Position of Film Hole Extraction in Internal Ribbed Cooling Channels of Turbine Blades

The concurrent use of film cooling and internal cooling plays an important role to maintain the life of turbine blades and increase thermal efficiency. Several studies were published on the interaction of these cooling strategies but these are mainly investigations on how internal cooling influences film cooling. The present study contributes to an improved understanding on how the cooling extraction through film cooling holes is influencing internal flow structures and therefore internal cooling. The flow field in an internal cooling channel is investigated by measuring the velocity distribution with 2D-PIV. Heat transfer measurements are performed using the thermochromic liquid crystal technique. The test stand models a rectangular cooling channel (AR=2:1), which is equipped with parallel ribs of four different geometries (90° ribs, 60° ribs, 60°-V-shaped ribs and 60°-Λ-shaped ribs). Bleed holes are placed in the rib segments and are positioned at three positions in streamwise direction. The suction ratio is varied between 0 and 6 and the cooling channel Reynolds number is 30.000.

Viscous Flows Driven by Uniform Shear over a Porous Stretching Sheet in the Presence of Suction/Blowing

An analysis is carried out to study the steady two-dimensional flow of an incompressible viscous fluid past a porous deformable sheet, which is stretched in its own plane with a velocity proportional to the distance from the fixed point subject to uniform suction or blowing. A uniform shear flow of strain rate β is considered over the stretching sheet. The analysis of the result obtained shows that the magnitude of the wall shear stress increases with the increase of suction velocity and decreases with the increase of blowing velocity and this effect is more pronounced for suction than blowing. It is seen that the horizontal velocity component (at a fixed streamwise position along the plate) increases with the increase in the ratio of shear rate β and stretching rate (c) (i.e., β/c) and there is an indication of flow reversal. It is also observed that this flow reversal region increases with the increase in β/c.

Streamwise variation of turbulent dynamics in boundary layer flow of drag-reducing fluid

Direct numerical simulations (DNSs) of a zero-pressure-gradient boundary layer flow of a polymeric fluid have been performed. The FENE-P model was used for the polymer stresses and a wide range of Weissenberg numbers ($\mathit{We}$) was addressed. In all cases, the streamwise variations of the level of polymer stretching and the level of drag reduction are anticorrelated. Consistent with earlier studies, the inlet condition for the flow consists of Newtonian velocity data with no polymer stretching, so in the upstream region of the boundary layer the polymer molecules stretch strongly in response, leading to an initial spatial maximum in polymer stretching. Beyond this initial region, the level of drag reduction increases with increasing downstream position, while the polymer stretch is decreasing. At sufficiently high Weissenberg numbers, these variations are monotonic with streamwise position (outside the upstream region), but at $\mathit{We}= 25$, both the polymer stretching and level of drag reduction display a decaying oscillation in the downstream position. The streamwise dependence of the velocity statistics is also shown. In addition, simulations in which the polymer stress is turned off beyond a chosen downstream position were performed; in this case the flow continues to exhibit substantial drag reduction well downstream of the cutoff position. These observations are analysed in light of other recent literature and in particular the observations of ‘active’ and ‘hibernating’ turbulence recently found in minimal channel flow by Xi and Graham. All of these observations suggest that an important role for viscoelasticity in the turbulent drag reduction phenomenon, at least near solid surfaces, is to suppress conventional turbulence, while leaving behind a much weaker form of turbulence that can persist for a substantial length of time (or downstream distance) even in the absence of viscoelastic stresses.

Simple three-dimensional vortex motions in coflowing jets and wakes

The vortex patterns which occur in coflowing jets and wakes at moderate Reynolds numbers (of order 500) are examined in detail. Flow visualization is used in conjunction with a flying hot-wire system which allows instantaneous velocity vector fields to be rapidly measured and related to the smoke patterns. The structures were made perfectly periodic in time by artificial stimulation. The experiments were therefore completely deterministic. This newly developed data-acquisition technique does not require the use of Taylor's hypothesis for inferring patterns from a fixed streamwise position. It therefore allows the vector fields of rapidly evolving patterns to be produced. It also allows the phenomenon of three-dimensional vortex pairing to be studied. The classification of patterns and conjectured topologies made by Perry & Lim (1978a, b) and the interpretations of Perry, Lim & Chong (1980) and Perry & Watmuff (1981) are examined. In the light of more-detailed measurements, it is found that some of these interpretations require modification.Vortex ‘skeleton’ models of the patterns are constructed and it is found that the Biot-Savart law gives vector fields which are in reasonable agreement with the experiments. It is also found that the power-spectral density of a periodic array of eddies can be explained in terms of potential flow vortices.