327lec-drag - 530.327 - Introduction to Fluid Mechanics -...

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Unformatted text preview: 530.327 - Introduction to Fluid Mechanics - Su Boundary layers: Pressure gradients and separation. Reading: Text, 9.6 and 9.7. You will remember from our earlier discussion of inviscid ows that steady, uniform, incompressible ow of inviscid uid past a body resulted in no drag on the body, even though in the real world we know that putting a body in a ow gives us drag. We called this DAlemberts Paradox. At the time, we learned that the proper handling of drag forces in such a situation requires consideration of viscosity. In class we decomposed the drag force into surface friction, which has to do with viscous shear stresses, and pressure forces. Its obvious, then, that the viscosity of a uid leads to surface friction. Whats not so obvious is that the uid viscosity also explains the pressure forces. The explanation for this lies in the concept of boundary layer separation that we talked about earlier in the context of the first lab. Well start by looking at the mechanisms by which a boundary layer either remains attached to a surface, or separates. A boundary layer can remain attached to a surface if the ow streamlines in the boundary layer have no diculty following the surface contours. To understand what might cause the boundary layer to be unable to follow the surface contours, consider Fig. 1, depicting the boundary layer on a at plate, where the mean ow is from left to right. In the leftmost cartoon, Figure 1: Representation of boundary layer separation. the ow is moving in the positive x-direction, pushed along by the negative pressure gradient, dp/dx < 0. The velocity gradient at the surface, du/dy ( y = 0), is positive. Now suppose we increase the pressure gradient (i.e. make it more positive). As the pressure gradient changes sign and becomes positive, it will want to push the ow in the negative x-direction. If the ow has some inertia, it will tend to push against this adverse pressure gradient. At some point, these effects will be in balance, and the situation in the middle cartoon in Fig. 1 will pertain, where du/dy at the surface is zero. As the pressure gradient is increased past this point, we get ow reversal near the surface (the cartoon on the right). When this happens, the left-to-right boundary layer ow can no longer follow the contour of the surface. Instead, the boundary layer is diverted upward to go around the region of reversed ow. This process is calledaround the region of reversed ow....
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This note was uploaded on 04/07/2008 for the course MECHENG 327 taught by Professor Su during the Spring '08 term at Johns Hopkins.

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327lec-drag - 530.327 - Introduction to Fluid Mechanics -...

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