![]() You might think the box was empty when you poured all the water out, but it was not. * There are a number of different ways to define a Reynolds number.Dry off your workspace and the glass. This is really important for sediment transport, and we'll talk more about it in a few minutes. Laminar flow also occurs locally in turbulent flows right at the contact between the fluid and a smooth surface because u becomes very slow. Also, very thin, slow flows of water, such as water flowing off a smooth cement parking lot, have low Re because l and u are small. In contrast, ice has a high µ and flows slowly ( u is low), so it is usually laminar. Rivers and wind storms are good examples of turbulent flow. In most cases, water and air flows have high Re because l is large, u is high and µ is low. Turbulent flow has Re greater than 2000 and laminar flow has Re less than 500.* Flow with Re between 5 is transitional and has some characteristics of laminar flow, but some turbulence as well. The magnitude of Re gives an idea of whether the flow is turbulent or laminar. Thus, a flow with a high viscosity (ice) tends to have less turbulence than a low viscosity flow (air). In contrast, viscous forces tend to suppress turbulence by damping out variations in motion through friction. Thus, high inertial forces tend to cause more turbulence. Inertia is the resistance to change in motion, and inertial forces tend to make a bit of the fluid keep flowing in its own direction if it is misdirected from the main flow direction. Re can be viewed as inertial forces divided by viscous forces. These all cancel out to form a unitless number, if you choose the same set of units for each variable, which you should always do. The units for this equation are typically (length)*(length/time)*(mass/length 3 )/(mass/(length*time)). Re = (fluid inertial forces)/(fluid viscous forces) = l*u* ρ /µ. The book uses µ/ ρ = v (kinematic viscosity). The variables for the Reynolds number ( Re) are: flow velocity ( u), characteristic length ( l) which represents flow geometry, say river depth, fluid density ( ρ), and fluid viscosity ( µ). It has a high viscosity relative to water and air (but low compared to most rock).īack to the Reynolds number. Ice is more viscous and impossible to move through because of the crystal bonds between the water molecules. This is due to the “friction” between adjacent water molecules, i.e. Swimming is more difficult because the water drags on your body. Walking through air is easy, because there is not much friction between air molecules. Viscosity is sort-of like the amount of friction within a substance. Viscosity is a measure of the resistance of a material to flow, i.e. The Reynolds number predicts the extent of turbulence in a fluid based on how fast the fluid is flowing, the geometry of the flow (how deep and wide it is, etc.), and the density and viscosity the of the fluid. Is the flow in glaciers laminar or turbulent? How can you tell? What properties of ice make it behave differently than water in terms of the amount of turbulence? The image to the right is of a glacier in Pearse Valley, Antarctica. The suspended sediment in the brown river is mixed into the clear water of the other river downstream. The image to the left is of two rivers in Costa Rica mixing turbulently. Movies of Laminar and Turbulent Flow: YouTube Fluid Dynamics Playlist, YouTube Fluid Dynamics Playlist 2018 (for videos shown in class) For example, dye may take some time to mix into the flow, but it does mix. Transitional Flow– Transitional flows have some characteristics of laminar flow and some of turbulent flow. Turbulence is important for sediment transport in water and air because it makes grains easier to transport and tends to keep them moving longer. Most water and air flows are turbulent, at least to some degree. There is abundant mixing in the flow because neighboring molecules move in different directions, and an added dye mixes into the water very quickly. Molecules move in all directions in bursts of upward, downward, and forward motion, and even some backward movement. Turbulent Flow- In contrast, turbulent flow is characterized by complex motion of water (or other) molecules. It is also characteristic of flows in "fluids" that are very viscous, like glacial ice or mud flows that have little water. Laminar flow is characteristic of very slow moving, shallow water, which is uncommon in nature. If you add a dye to water that is in the laminar flow regime, the dye would not mix into the water it would streak out in an approximately straight line. Laminar Flow- In laminar flow, water molecules move in straight, parallel lines down current. There is a wide gradation between these two end members, specifically "transitional" flows. ![]() There are two end member ways fluids flow: 1) laminar flow and 2) turbulent flow. ![]()
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