Abstract
For the theoretical consideration of a system for reducing skin friction, a mathematical model was derived to represent, in a two-phase field, the effect on skin friction of the injection of micro air bubbles into the turbulent boundary layer of a liquid stream. Based on the Lagrangian method, the equation of motion governing a single bubble was derived. The random motion of bubbles in a field initially devoid of bubbles was then traced in three dimensions to estimate void fraction distributions across sections of the flow channel, and to determine local bubble behavior. The liquid phase was modeled on the principle of mixing length. Assuming that the force exerted on the liquid phase was equal to the fluid drag generated by bubble slip, an equation was derived to express the reduction in turbulent shear stress. Corroborating experimental data were obtained from tests using a cavitation tunnel equipped with a slit in the ceiling from which bubbly water was injected. The measurement data provided qualitative substantiation of the trend shown by the calculated results with regard to the skin friction ratio between cases with and without bubble injection as function of the distance downstream from the point of bubble injection.
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Abbreviations
- B :
-
law of wall constant
- C f :
-
local coefficient of skin friction
- C f0 :
-
local coefficient of skin friction in the absence of bubbles
- d b :
-
bubble diameter [m]
- g :
-
acceleration of gravity [m/s2]
- k 1∼ k4 :
-
proportional coefficient
- k L :
-
turbulent energy of the liquid phase [m2/s2]
- L :
-
representative length [m]
- l b :
-
mean free path of a bubble [m]
- m A :
-
added mass of a single bubble [kg]
- m b :
-
mass of a single bubble [kg]
- N x ,N y ,N z :
-
force perpendicular to the wall or ceiling exerted on a bubble adhering to that wall or ceiling [N]
- P :
-
absolute pressure [Pa]
- Q G :
-
rate of air supply [ℓ/min]
- q ′(i) L :
-
turbulent velocity at the ith time increment [m/s]
- R> ex :
-
Reynolds number defined by Eq. 32
- T *L :
-
integral time scale of the liquid phase [s]
- U :
-
velocity of the main stream [m/s]
- ū,¯v,¯w :
-
time-averaged velocity components [m/s]
- u′,v′,w′ :
-
turbulent velocity components [m/s]
- û L ′,vL′:
-
root mean square values of liquid phase turbulence components in thex- and y-directions [m/s]
- V :
-
volume of a single bubble [m3]
- X,Y,Z :
-
components of bubble displacement [m]
- x s ,y s ,z s :
-
coordinate of a random point on a sphere of unit diameter centered at the coordinate origin
- Ŷ :
-
root mean square of bubble displacement in they-direction in reference to the turbulent liquid phase velocity [m]
- α :
-
local void fraction
- α m :
-
mean void fraction in a turbulent region
- γ :
-
regular random number
- ΔR v :
-
increment of the horizontal component of the force acting on a single bubble, defined by Eq. 22 [N]
- Δt :
-
time increment [s]
- Δɛ1 :
-
reduction of turbulent stress [N/m2]
- ε L :
-
rate of liquid energy dissipation [m2/s3]
- η m :
-
coefficient defined by Eq. 30
- κ :
-
law of wall constant in the turbulent region in absence of bubbles
- κ 1 :
-
law of wall constant in the turbulent region in presence of bubbles
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Yoshida, Y., Takahashi, Y., Kato, H. et al. Simple Lagrangian formulation of bubbly flow in a turbulent boundary layer (bubbly boundary layer flow). J Mar Sci Technol 2, 1–11 (1997). https://doi.org/10.1007/BF01245932
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DOI: https://doi.org/10.1007/BF01245932