Skip to content
Sahithyan's S1
Sahithyan's S1 — Fluid Mechanics

Hydrostatic Thrust

On a Plane Surface

Acts normal to the surface on the centre of pressure with a magnitude of:

Thrust=submerged area×Pc\text{Thrust} = \text{submerged area} \times P_c

CC is the centroid of the submerged area. PcP_c is the pressure at the centroid.

Centre of Pressure

yp=yc+IccAycy_p = y_c + \frac{I_{\text{cc}}}{A\cdot y_c}

Here:

  • AA - Total submerged area
  • ypy_p - Distance to centre of pressure measured along the submerged surface from the free surface
  • ycy_c - Distance to CC measured along the submerged surface from the free surface
  • IccI_{\text{cc}} - Second moment of submerged area about the centroidal axis parallel to the free surface

For common shapes

ShapeDescriptionypy_p
ParallelogramBase bb. Height hh. Base is at the free surface.2h3 \cfrac{2h}{3}
TriangleBase bb. Height hh. Base is at the free surface.5h6\cfrac{5h}{6}
CircleRadius rr. Center is at a depth rr.5r4\cfrac{5r}{4}

Proof

Hydrostatic thrust on a plane surface

Direction

All forces acting on the surface is normal to the surface. Therefore FF is normal to the surface.

Magnitude

F=AdF=ApdA=AysinθρgdAF = \int_A{\text{d}F} = \int_A{p\text{d}A} = \int_A{y\sin{\theta}\rho g\, \text{d}A} F=sinθρgAydA=sinθρgAyc=AycsinθρgF = \sin{\theta}\rho g \int_A{y\,\text{d}A} = \sin{\theta}\rho g \cdot A y_{c} = A\cdot {y_{c}\sin{\theta}\rho g} F=APcF = AP_c

Line of action

Fyp=AydFF \cdot y_p = \int_{A}{y\,\text{d}F} yp=AydFAdF=Ay(ysinθρg)dAAysinθρgdA=Ay2dAAydAy_p = \frac{\int_{A}{y\,\text{d}F}}{\int_{A}{\text{d}F}} = \frac{\int_{A}{y(y\sin{\theta}\rho g)\,\text{d}A}}{\int_{A}{y\sin{\theta}\rho g\,\text{d}A}} = \frac{\int_{A}{y^2\,\text{d}A}}{\int_{A}{y\,\text{d}A}} yp=IooAyc=yc+IccAycy_p = \frac{I_{oo}}{Ay_c} = y_c + \frac{I_{cc}}{Ay_c}

On a Curved Surface

Fx=Thrust exerted on the vertical projection of the submerged surfaceF_x = \text{Thrust exerted on the vertical projection of the submerged surface} Fy=Weight of the fluid above submerged surfaceF_y = \text{Weight of the fluid above submerged surface}

Proof

Hydrostatic thrust on a curved surface

For the equilibrium of the fluid volume ABCDAABCDA.

Fy=WABCDAF_y = W_{ABCDA}

For the equilibrium of the fluid volume ABEAABEA.

Fx=FAEF_x = F_{AE}