Flow Visualization in a Water Channel - Fluid Flow - Lab Manual, Study notes of Fluid Dynamics

Download Flow Visualization in a Water Channel - Fluid Flow - Lab Manual and more Study notes Fluid Dynamics in PDF only on Docsity! Flow Visualization in a Water Channel Nomenclature A projected frontal area (for the sphere) or planform area (for the airfoil) c chord length of an airfoil or wing CD drag coefficient: CD = 2FD/V2 CL lift coefficient: CL = 2FL/V2 d diameter of a cylinder, sphere, or other object FD drag force FL lift force g gravitational constant (9.81 m/s2) h head: elevation of a fluid column or height difference between two liquid columns P static pressure Red Reynolds number based on diameter d: Red = Vd/ T temperature V mean freestream velocity in a pipe z elevation in vertical direction  angle of attack of an airfoil  average surface roughness height in a pipe or on the surface of a body  coefficient of dynamic viscosity (also called simply the viscosity)  coefficient of kinematic viscosity  density of the fluid Educational Objectives 1. Develop familiarity with operation of a small closed-loop water channel, a laser velocimetry system, and flow visualization by dye injection. 2. Visualize flow separation, and reinforce the concepts of Reynolds number, drag crisis, and the effect of surface roughness on a boundary layer. 3. Visualize flow over a model automobile, and examine whether the flow separates or not. 4. Visualize and reinforce the concept of airfoil stall. Equipment 1. closed-loop water channel, built by Engineering Laboratory Design, Inc. 2. Dantec FlowLite fiber optic laser velocimeter (LV) system, with software and traversing system 3. IBM-compatible personal computer 4. spheres of various sizes, some with roughness, sting mounted from a top cover plate:  Golf ball d = 1.68 inches (rough)  Yellow-orange ball d = 2.83 inches (smooth)  Yellow-orange ball, roughened d = 2.83 inches (rough)  Baseball d = 2.84 inches (rough) Note: These spheres are interchangeable with those used with the wind tunnel experiment. The threads on the sting mount are 1/4 - 20, just as in the wind tunnel. 5. Model car(s) 6. NACA 0012 airfoil, with 4.0 inch chord length and adjustable angle of attack, with attached tufts, mounted from a top cover plate 7. dye, dye reservoirs, dye injection needles, and backlight system (the dye is standard household food coloring, mixed in a ratio of one part concentrated food coloring to 10 parts water) docsity.com Background This experiment complements the wind tunnel experiment in which the drag of a sphere in a freestream is investigated. In that experiment, the concept of “drag crisis” is observed. Specifically, at a Reynolds number (based on sphere diameter and freestream velocity, Red = Vd/) somewhere between 2  105 and 3  105, the drag suddenly drops. This drag crisis is due to the sudden change from laminar boundary layer separation to turbulent boundary layer separation along the surface of the smooth sphere. Since a turbulent boundary layer is much more resistant to flow separation than is a laminar boundary layer, the turbulent boundary layer clings to the surface of the sphere much further downstream before separating. Thus, the wake is narrower and the pressure drag is greatly reduced, and so is the overall drag. Oftentimes, especially in sports, roughness is intentionally added to the surface of a ball in order to “trip” the boundary layer to become turbulent, and hence to decrease the drag. This concept seems to be well-known, but what many people (including many ME 33 graduates!) do not realize is that surface roughness does not always induce a decrease in drag! If the Reynolds number is too low, the boundary layer cannot be tripped, and remains laminar in spite of the roughness. In Figure 1, taken from Reference 1, the effect of surface roughness  on drag coefficient CD is shown. Clearly, as roughness height increases, the Reynolds number at which the boundary layer changes from laminar to turbulent decreases. Flow disturbances due to the surface roughness cause premature transition of the boundary layer from laminar to turbulent, and the drag crisis is forced to occur at a lower Reynolds number. However, the Reynolds number must still be relatively high for the roughness to be effective, even for large values of . Note for example that at Reynolds numbers below about 4  104, the flow over a golf ball remains laminar (and the drag remains high) in spite of the severe surface roughness. Figure 1. Effect of surface roughness on the drag coefficient of a sphere. As mentioned above, the wake of a sphere at Reynolds numbers above the drag crisis is narrower than that of the sphere at pre-transition Reynolds numbers. This is illustrated in Figure 2. Laminar boundary layer separation; wide wake. Turbulent boundary layer separation; narrow wake. Figure 2. Comparison of the wakes of a sphere with laminar and turbulent boundary layer separation. Such a significant change in flow separation location and wake width should be easily identifiable with flow visualization. In this lab experiment, you will attempt to visualize this phenomenon by injecting dye streaks into the flow of water over a sphere. In addition to the drag crisis on spheres, boundary layer flow separation is important in many other practical flow fields as well. One well-known example is the “stall” of an aircraft wing at high angles of attack. The purpose of an airfoil- shaped wing is to generate lift with as little drag as possible. When an airfoil is tilted to an angle of attack  of a few degrees, the fluid travels faster over the upper wing surface than over the lower surface. By Bernoulli's principle, the pressure on the upper surface is therefore less than that on the lower; a lifting force with very little drag is the result. As angle of attack is docsity.com