Showing papers by "Mohamed Gad-el-Hak published in 1990"

Control of low-speed airfoil aerodynamics

Abstract: Nomenclature CD = drag coefficient ( = 2D/pUlc) Cf = local skin-friction coefficient ( = 2rw/pUl) CL = lift coefficient ( = 2L/pU^c) Cq = suction coefficient ( = I vw I / UQ) c = airfoil's chord D = drag force per unit span L = lift force per unit span P = instantaneous hydrostatic pressure PO = pressure outside boundary layer P = mean pressure R = wall's radius of curvature /?6* = displacement thickness Reynolds number (= Uod*/v) RO = momentum thickness Reynolds number ( = U0de/v) Re = Reynolds number based on distance from leading edge (or chord) and freestream velocity T = instantaneous temperature T = mean temperature J7, = instantaneous velocity component Uj = mean velocity component U0 = velocity outside the boundary layer t/oo = freestream velocity HI = fluctuating velocity component u* = friction velocity ( = Vr^/p) vw = normal velocity of fluid injected or withdrawn through the wall X-, = Cartesian coordinates x = streamwise distance from leading edge y = normal distance from the wall y = normal distance in wall units ( = yu*/v) Z = spanwise coordinate a. = angle of attack 6 = boundary-layer thickness 60 = momentum thickness 6* = displacement thickness /x = dynamic coefficient of viscosity v = kinematic viscosity v/u * = viscous length scale (wall unit) p = density — p~uv = tangential Reynolds stress T.W = shear stress at the wall ( = pu *) [AJo = instantaneous spanwise vorticity at the wall _ [fijo = rnean spanwise vorticity at the wall (= — [dU/dy]Q)

Low‐Reynolds number flow over a rotatable cylinder–splitter plate body

Abstract: Uniform flow around a body consisting of a cylinder with a splitter plate attached to it is considered. The body is free to rotate about the axis of the cylinder. Numerical results show that on increasing the Reynolds number above a critical value a symmetry‐breaking bifurcation appears and the splitter plate migrates to an asymmetric equilibrium position, confirming previous high‐Reynolds number experiments. The present results reveal that this phenomenon is due to the flow in the separation bubble behind the cylindrical part of the body.

Flow Control by Suction

Abstract: A viscous fluid which is initially irrotational will acquire vorticity when an obstacle is passed through the fluid. This vorticity controls the nature and structure of the boundary layer flow. For an incompressible, two-dimensional, wall-bounded flow, the flux of spanwise vorticity at the wall, and hence whether the surface is a sink or a source of vorticity, is affected by the wall modon (e.g. in the case of a compliant coating), transpiration (suction or injection), pressure gradient in the ambient flow, wall curvature, and viscosity gradient near the wall (e.g., heating/cooling of the wall or introducing a shear-thinning additive into the boundary layer). These alterations separately or collectively control the shape of the mean velocity profile which in turn determines the skin friction at the wall, the boundary layer ability to resist transition and separation, and the intensity of turbulence and its structure. In the case of a turbulent flow, the situation is complicated in that modulations such as pressure gradient or suction can dramatically change the production of Reynolds stress in the wall region and hence the momentum transport in the boundary layer.