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Turbulent vs laminar flow9/5/2023 ![]() ![]() In turbulent flow vortices, eddies and wakes make the flow unpredictable. Shear stress in a laminar flow depends almost only on viscosity - μ - and is independent of density - ρ. Laminar flow can be regarded as a series of liquid cylinders in the pipe, where the innermost parts flow the fastest, and the cylinder touching the pipe isn't moving at all. Laminar flow generally happens when dealing with small pipes and low flow velocities. To delay separation, to promote mixing, to avoid uncertainties.There are in general three types of fluid flow in pipes REASONS TO PREFER TURBULENT BOUNDARY LAYER Surface roughness, Surface curvature, flow divergence, heat transfer, mass transfer, Structural vibration, Noise. Pressure gradients outside boundary layer. Generally, in addition to the flow being efficient you want it to be predictable and repeatable, so you may actually prefer turbulent flow.ĪDDED (PARTIAL LIST OF "OTHER THINGS" TO BE TAKEN INTO ACCOUNT) In the real world there are insects, dirt and raindrops that tend to promote turbulence. In some proposals the boundary layer is removed by sucking it through the surface, which requires the suction ports to stay open. There have been many attempts to design aircraft whose wings will support laminar flow and it is possible to do this under ideal conditions where the wing surface can be kept clean and smooth. When turbulence does increase friction, the basic mechanism is that turbulent mixing inside the boundary layer brings fast-moving air down toward the surface, creating a larger velocity gradient (and therefore more friction) at the surface. However, the list of "other things"is quite long. Turbulent flow gives more (Added as edit LOCAL) friction drag other things being equal. Skin friction is much more important for a wing, so a design that maintains a laminar flow wins over a design that results in a turbulent flow. Form drag is, by design, a minor contributor to the total drag force. The total drag on a dimpled ball is much less than on an undimpled ball. Skin friction increases somewhat, but pressure drag is significantly smaller for a dimpled vs undimpled golf ball. This in turn moves the separation point moves well toward the rear of the ball. The dimples make the flow turbulent over almost all of the surface. This separation occurs early in a smooth ball, resulting in a rather large form drag. The flow remains laminar over a good portion of a smooth ball, up to the point where the flow separates from the ball. Form drag (pressure drag) dominates over skin friction in the case of a golf ball. You need to look at all aspects of drag, and these vary dramatically based on object shape, surface texture, and speed.Ī golf ball has dimples because skin friction is a small component of drag for blunt objects. This is but one component of parasitic drag, which in turn is but one component of the total drag on an object. This results in a thinner laminar boundary layer which, relative to laminar flow, depreciates the magnitude of friction force as fluid flows over the object. While the turbulent layer grows, the laminar layer thickness decreases. Turbulent flow has a fluctuating and irregular pattern of flow which are made obvious by the formation of vortices. At some point along the flow direction, the flow becomes unstable and becomes turbulent. As a fluid flows over a surface shear stresses within the fluid slow additional fluid particles causing the boundary layer to grow in thickness. The flow over a body may begin as laminar. The laminar boundary is a very smooth flow, while the turbulent boundary layer contains swirls or "eddies." The laminar flow creates less skin friction drag than the turbulent flow, but is less stable. Is friction drag more or less in laminar boundary layer vs turbulent boundary layer? ![]()
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