Knowledge card
Momentum ODE
Mathematical Modelling — Fluid Dynamics & Heat TransferContinuum mechanics; boundary-layer similarity theory
Symbol
The third-order similarity-reduced streamwise momentum equation; its formula is composed on demand from the Powell–Eyring, unsteady, magnetic, and buoyancy terms.
About levels
Why the levels load one at a time
Only level 0 travels with this article; the expansion runs to level 9. The complete expansion is several megabytes, so deeper levels are fetched one at a time when you ask for them.
To go deeper still, open any quantity in the reading as its own concept card — it carries its own derivation and its own depth ladder.
In words:equals(plusminusplus)divided by.
Open full derivation chain: 5 stepsDerivation
- 1
Open Step 1: Given / definition · The derived Momentum PDE (equation 3.24) as the given definition for reduction: unsteady accele…
The derived Momentum PDE (equation 3.24) as the given definition for reduction: unsteady acceleration plus convective acceleration equals the Powell–Eyring stress terms plus thermal buoyancy minus the Lorentz body force — five relational blocks.Given / definition Defining ruleThe starting relation of a derivation — the definition or established equation every later step transforms; it justifies no change itself.Level 0 · more availableBrief — named quantitiesAbout levels
Why the levels load one at a time
Only level 0 travels with this article. The complete expansion is several megabytes, so deeper levels are fetched one at a time when you ask for them — and how deep it goes is not known until you get there.
To go deeper still, open any quantity in the reading as its own concept card — it carries its own derivation and its own depth ladder.
How many levels remain is not yet known — the depth is discovered one level at a time.
- 2
Open Step 2: Similarity transformation · Substitute the similarity maps into EVERY block [eq. 3.40]: u=U_wf′, v=−√(cν_f/(1−γt))f, and T−…
Substitute the similarity maps into EVERY block [eq. 3.40]: u=U_wf′, v=−√(cν_f/(1−γt))f, and T−T∞=(T_r−T∞)θ with T_r−T∞=ΔT₀x/(1−γt)².Similarity transformation Defining ruleu(x, y, t) = U(x, t)·F(η), η = y/δ(x, t)A change of variables that collapses several independent variables into one similarity coordinate, reducing a PDE to an ODE; each substituted term splits into a dimensional scale times a dimensionless group.Substitution · η Operand nodeSimilarity Variable ηThe self-similar wall-normal coordinate that collapses the boundary layer; its definition is composed on demand from the rate constant, viscosity, and time.Substitution: The operand (a map or model) replaces the source directly.Open operand record: Substitution η
Substitution:L0L2Open term-change ledger: 5 records
Open record 1: ∂u/∂t → γ(cx/(1−γt)²)(f′+f″(η/2))
Consumes:L0L1Produces:L0L1The similarity transform inserts u = U_w f′(η) into the local acceleration ∂u/∂t; carrying out the time derivative (detailed in the Substituted Unsteady Acceleration node) turns the block into the growth scale cx/(1−γt)² times the transient velocity group f′+f″(η/2).γ(cx/(1−γt)²)(f′+f″(η/2))Resulting expressionSubstituted Unsteady AccelerationThe unsteady acceleration after the similarity substitution — the growth scale times the transient velocity group.Open record 2: u·∇u → (c²x/(1−γt)²)(f′²−ff″)
Consumes:L0L1Produces:L0L1The similarity transform inserts u = U_w f′(η) and v = −√(cν_f/(1−γt))f(η) into the convective terms u∂u/∂x and v∂u/∂y, turning the block into the convective scale c²x/(1−γt)² times f′²−ff″ (detailed per-term in the Substituted Convective Acceleration node).(c²x/(1−γt)²)(f′²−ff″)Resulting expressionSubstituted Convective AccelerationThe convective acceleration after the similarity substitution — its streamwise (f′²) and wall-normal (ff″) parts grouped as ONE block, so the balance keeps the same convective-acceleration concept it had before substitution. Derives from the convective acceleration.Open record 3: τ → τ_PE(η)
Consumes:L0L2Produces:L0L1The similarity transform inserts the shear-rate and curvature maps ∂u/∂y = U_w(c/(ν_f(1−γt)))^{1/2}f″(η), ∂²u/∂y² = U_w(c/(ν_f(1−γt)))f‴(η) into the Powell–Eyring diffusion and retardation terms (detailed step-by-step in the Substituted Powell–Eyring Stress Terms node), exposing their dimensional coefficients explicitly. Consumed at DEPTH 2 so the second-order velocity gradients (∂²u/∂y², (∂u/∂y)²) that the maps act on are visible, not the opaque τ symbol.τ_PE(η)Resulting expressionSubstituted Powell–Eyring Stress TermsThe Powell–Eyring stress after substitution — its linear diffusion and nonlinear retardation parts grouped as ONE block, preserving the Powell–Eyring stress concept across the substitution. Derives from the Powell–Eyring stress terms.Open record 4: ρgβ → gβ_hnf(Tr−T∞)θcosα
Consumes:L0L1Produces:L0L1The similarity transform inserts T−T∞=(T_r−T∞)θ with T_r−T∞=ΔT₀x/(1−γt)², preserving a temperature scale distinct from the velocity scale.gβ_hnf(Tr−T∞)θcosαResulting expressionSubstituted BuoyancyThe buoyancy after substituting T−T∞=(T_r−T∞)θ; its reference excess is a temperature, so the term remains an acceleration.Open record 5: J×B → (σ_hnf B²/ρ_hnf)U_w f′
Consumes:L0L1Produces:L0L1The similarity transform inserts u = U_w f′(η) into the Lorentz body force, turning it into the magnetic coefficient times U_w f′ (detailed in the Substituted Lorentz Braking node); B(t) remains time-dependent at this stage.(σ_hnf B²/ρ_hnf)U_w f′Resulting expressionSubstituted Lorentz BrakingThe magnetic body force after substituting the velocity map — B(t) still time-dependent at this stage.Level 0 · more availableBrief — named quantitiesAbout levels
Why the levels load one at a time
Only level 0 travels with this article. The complete expansion is several megabytes, so deeper levels are fetched one at a time when you ask for them — and how deep it goes is not known until you get there.
To go deeper still, open any quantity in the reading as its own concept card — it carries its own derivation and its own depth ladder.
How many levels remain is not yet known — the depth is discovered one level at a time.
- 3
Open Step 3: Rearrange terms · Move every term to one side and set the equation to zero [eq. 3.42], the standard homogeneous f…
Move every term to one side and set the equation to zero [eq. 3.42], the standard homogeneous form: subtracting the stress and buoyancy blocks and adding the Lorentz block collects the whole balance against zero — the SAME five blocks, now equated to nought.Rearrange terms Defining rulea + b = c ⟺ a = c − bMoving terms across an equality by applying the same operation to both sides; the solution set is unchanged.Level 0 · more availableBrief — named quantitiesAbout levels
Why the levels load one at a time
Only level 0 travels with this article. The complete expansion is several megabytes, so deeper levels are fetched one at a time when you ask for them — and how deep it goes is not known until you get there.
To go deeper still, open any quantity in the reading as its own concept card — it carries its own derivation and its own depth ladder.
How many levels remain is not yet known — the depth is discovered one level at a time.
- 4
Open Step 4: Non-dimensionalisation · Divide by c²x/(1−γt)² and identify each raw coefficient: γ/c=A; (ν_hnf+1/(ρ_hnfβς))/ν_f=(φ_μ+ε)…
Divide by c²x/(1−γt)² and identify each raw coefficient: γ/c=A; (ν_hnf+1/(ρ_hnfβς))/ν_f=(φ_μ+ε)/φ_ρ; retardation=εδ/φ_ρ; gβ_hnfΔT₀/c²=(φ_{ρβ}/φ_ρ)λ with λ=Gr_x/Re_x²=gβ_fΔT₀/c²; and magnetic damping=(φ_σ/φ_ρ)M. Every explicit x and t cancels.Non-dimensionalisation Defining rulex* = x/x_ref ⟹ dimensionless groups (Π-theorem)Scaling every variable by a reference quantity so the equation is left with named dimensionless groups as its only coefficients.Divisor · c²x/(1−γt)² Operand nodeConvective Momentum ScaleThe dimensional scale of u∂u/∂x — the common factor every momentum term carries after substitution, whose cancellation IS the self-similarity.Divisor: The source is divided by the operand, leaving a named remainder.Open operand record: Divisor c²x/(1−γt)²
Divisor:L0L1Open term-change ledger: 5 records
Open record 1: γ(cx/(1−γt)²)(f′+f″(η/2)) → A(f′+(η/2)f″)
Consumes:L0L1Produces:L0L1Dividing the unsteady block by the convective scale c²x/(1−γt)² leaves the raw ratio γ/c, identified BY DEFINITION as the unsteadiness parameter A — giving the unsteadiness term A(f′+(η/2)f″).A(f′+(η/2)f″)Resulting expressionUnsteadiness TermThe transient momentum term — the unsteadiness parameter times its velocity group.Open record 2: (c²x/(1−γt)²)(f′²−ff″) → f′²−ff″
Consumes:L0L1Produces:L0L1Dividing the convective block by the same convective scale c²x/(1−γt)² cancels its common dimensional factor exactly — that cancellation IS the self-similarity — leaving the dimensionless inertia f′²−ff″.f′²−ff″Resulting expressionDimensionless Convective InertiaThe convective inertia of the reduced ODE — the ff″ and f′² terms grouped as ONE block, the dimensionless form of the substituted convective acceleration it derives from.Open record 3: τ_PE(η) → ((φ_μ + ε)/φ_ρ) f‴ − ((εδ/φ_ρ))f″²f‴
Consumes:L0L1Produces:L0L1Dividing the Powell–Eyring block by the convective scale exposes the raw diffusion and retardation coefficients, identified BY DEFINITION as (φ_μ+ε)/φ_ρ and εδ/φ_ρ (the verified grouping carrying 1/φ_ρ onto BOTH stress groups).((φ_μ + ε)/φ_ρ) f‴ − ((εδ/φ_ρ))f″²f‴Resulting expressionDimensionless Powell–Eyring TermsThe Powell–Eyring diffusion and nonlinear retardation of the reduced ODE grouped as ONE block — the dimensionless form of the substituted Powell–Eyring stress it derives from. The verified grouping carries 1/φ_ρ onto BOTH stress groups.Open record 4: gβ_hnf(Tr−T∞)θcosα → (φ_β/φ_ρ) λ θ cos α
Consumes:L0L1Produces:L0L1Using T_r−T∞=ΔT₀x/(1−γt)², division leaves gβ_hnfΔT₀/c². The explicit Gr_x and Re_x formulas identify this as (φ_{ρβ}/φ_ρ)λ with λ=gβ_fΔT₀/c².(φ_β/φ_ρ) λ θ cos αResulting expressionBuoyancy TermThe thermal buoyancy term — the expansion-to-density ratio times the mixed-convection parameter times the temperature times the inclination factor.Open record 5: (σ_hnf B²/ρ_hnf)U_w f′ → (φ_σ/φ_ρ) M f′
Consumes:L0L1Produces:L0L1Dividing the Lorentz block by the convective scale and inserting B(t)² = B₀²/(1−γt) cancels the residual (1−γt) exactly, identifying σ_hnf B₀²/(ρ_hnf c) BY DEFINITION as (φ_σ/φ_ρ)M.(φ_σ/φ_ρ) M f′Resulting expressionMagnetic Damping TermThe Lorentz braking term — the conductivity-to-density ratio times the magnetic parameter times the dimensionless velocity.Level 0 · more availableBrief — named quantitiesAbout levels
Why the levels load one at a time
Only level 0 travels with this article. The complete expansion is several megabytes, so deeper levels are fetched one at a time when you ask for them — and how deep it goes is not known until you get there.
To go deeper still, open any quantity in the reading as its own concept card — it carries its own derivation and its own depth ladder.
How many levels remain is not yet known — the depth is discovered one level at a time.
- ★
Open Step 5: Separation of variables · Isolate the highest derivative: factor f‴ out of the Powell–Eyring stress and divide the balanc…
Isolate the highest derivative: factor f‴ out of the Powell–Eyring stress and divide the balance by its coefficient, giving the explicit normal form f‴ = … that the numerical solver marches.Separation of variables Defining ruledy/dx = f(x)g(y) ⟹ ∫dy/g(y) = ∫f(x)dxSplitting a differential equation so each side carries one variable, then integrating both sides.Divisor · (φ_μ+ε)/φ_ρ−(εδ/φ_ρ)f″² Operand nodeEffective Diffusion DenominatorThe shear-dependent coefficient dividing the momentum forcing — it must stay positive for the model to remain parabolic.Divisor: The source is divided by the operand, leaving a named remainder.Open term-change ledger: 1 records
Open record 1: ((φ_μ + ε)/φ_ρ) f‴ − ((εδ/φ_ρ))f″²f‴ → f‴ + (φ_μ+ε)/φ_ρ−(εδ/φ_ρ)f″²
Consumes:L0Produces:L0L0The Powell–Eyring stress ((φ_μ+ε)/φ_ρ)f‴ − (εδ/φ_ρ)f″²f‴ = f‴·[(φ_μ+ε)/φ_ρ − (εδ/φ_ρ)f″²] separates into the common factor f‴ (isolated on the left as the highest derivative) and the effective-diffusion denominator K = (φ_μ+ε)/φ_ρ − (εδ/φ_ρ)f″² that the balance is divided by.Level 0 · more availableBrief — named quantitiesAbout levels
Why the levels load one at a time
Only level 0 travels with this article. The complete expansion is several megabytes, so deeper levels are fetched one at a time when you ask for them — and how deep it goes is not known until you get there.
To go deeper still, open any quantity in the reading as its own concept card — it carries its own derivation and its own depth ladder.
How many levels remain is not yet known — the depth is discovered one level at a time.
Equation Workspace
Set known values, calculate unknowns, inspect dependencies, and run numerical solves.
Calculator
Equation Workspace
Set known values, calculate unknowns, inspect dependencies, and run numerical solves.
Equation workspace
Set the values you know, then calculate the remaining quantities in Momentum ODE.
Domain Analysis
Unit status, dimensional trails, per-step checks, and custom unit expressions.
Domain Analysis
Unit status, dimensional trails, per-step checks, and custom unit expressions.
Step-by-step unit check (5 steps)
Step 1 given
Expand checked this step.Step 2 similarity_transform
Expand checked this step.Step 3 rearrange
Expand checked this step.Step 4 non_dimensionalise
Expand checked this step.Step 5 separation_of_variables
Expand checked this step.
Try your own — the unit calculator
Ever wondered whether a formula “adds up”? This tool answers one plain question: do the units match? Pick a couple of quantities (distance, time, a speed…), join them with ×, ÷, +, or =, and press Run. It works out the resulting unit — for example distance ÷ time gives a speed, LT⁻¹ (metres per second) — and flags anything that can’t be right, like adding a length to a time.
Reads as: f‴
“f‴” is analysed by expanding its full defining equation (its children and their relations), not as a bare symbol. Use Check this concept’s units above for the same result.
Mathematical Analysis
Object type, lawful operations, conditions, comparisons, and derivation-step checks.
Mathematical Analysis
Object type, lawful operations, conditions, comparisons, and derivation-step checks.
Check one step at a time (5 steps)
Step 1 — given
Step 2 — similarity transform
Step 3 — rearrange
Step 4 — non dimensionalise
Step 5 — separation of variables
Essence
S1The SAME balance after substitution: substituted unsteady plus substituted convective acceleration equals substituted Powell–Eyring stress plus substituted buoyancy minus substituted Lorentz braking.
- S1.1essenceThe sum of γ(cx/(1−γt)²)(f′+f″(η/2)) and (c²x/(1−γt)²)(f′²−ff″). [1.1]
- S1.2essenceThe sum of τ_PE(η) and gβ_hnf(Tr−T∞)θcosα. [1.1]
- S1.3essence(τ_PE(η) + gβ_hnf(Tr−T∞)θcosα) reduced by (σ_hnf B²/ρ_hnf)U_w f′. [1.1]
- S1.4essence(γ(cx/(1−γt)²)(f′+f″(η/2)) + (c²x/(1−γt)²)(f′²−ff″)) set equal to ((τ_PE(η) + gβ_hnf(Tr−T∞)θcosα) − (σ_hnf B²/ρ_hnf)U_w f′). [1.1]
S2The substituted balance in homogeneous form: substituted unsteady plus substituted convective acceleration minus substituted Powell–Eyring stress minus substituted buoyancy plus substituted Lorentz braking equals zero.
- S2.1essenceThe sum of γ(cx/(1−γt)²)(f′+f″(η/2)) and (c²x/(1−γt)²)(f′²−ff″). [1.1]
- S2.2essence(γ(cx/(1−γt)²)(f′+f″(η/2)) + (c²x/(1−γt)²)(f′²−ff″)) reduced by τ_PE(η). [1.1]
- S2.3essence((γ(cx/(1−γt)²)(f′+f″(η/2)) + (c²x/(1−γt)²)(f′²−ff″)) − τ_PE(η)) reduced by gβ_hnf(Tr−T∞)θcosα. [1.1]
- S2.4essenceThe sum of (((γ(cx/(1−γt)²)(f′+f″(η/2)) + (c²x/(1−γt)²)(f′²−ff″)) − τ_PE(η)) − gβ_hnf(Tr−T∞)θcosα) and (σ_hnf B²/ρ_hnf)U_w f′. [1.1]
- S2.5essence((((γ(cx/(1−γt)²)(f′+f″(η/2)) + (c²x/(1−γt)²)(f′²−ff″)) − τ_PE(η)) − gβ_hnf(Tr−T∞)θcosα) + (σ_hnf B²/ρ_hnf)U_w f′) set equal to 0. [1.1]
S3Isolate the highest derivative: factor f‴ out of the Powell–Eyring stress and divide the balance by its coefficient, giving the explicit normal form f‴ = … that the numerical solver marches.
- S3.1essenceThe sum of A(f′+(η/2)f″) and f′²−ff″.
- S3.2essence(A(f′+(η/2)f″) + f′²−ff″) reduced by (φ_β/φ_ρ) λ θ cos α.
- S3.3essenceThe sum of ((A(f′+(η/2)f″) + f′²−ff″) − (φ_β/φ_ρ) λ θ cos α) and (φ_σ/φ_ρ) M f′.
- S3.4essence(((A(f′+(η/2)f″) + f′²−ff″) − (φ_β/φ_ρ) λ θ cos α) + (φ_σ/φ_ρ) M f′) divided by (φ_μ+ε)/φ_ρ−(εδ/φ_ρ)f″².
- S3.5essencef‴ set equal to ((((A(f′+(η/2)f″) + f′²−ff″) − (φ_β/φ_ρ) λ θ cos α) + (φ_σ/φ_ρ) M f′) / (φ_μ+ε)/φ_ρ−(εδ/φ_ρ)f″²).
Dimension I
Definition
What it is — and what it is not
- What it is
- S4The similarity-reduced streamwise momentum equation.
- S5The momentum balance: unsteady plus convective acceleration equals Powell–Eyring stress plus buoyancy minus the Lorentz force.
- Wisdoms
- S6The reduced momentum ODE in named dimensionless parameters: the unsteadiness term plus the dimensionless convective inertia minus the Powell–Eyring diffusion/retardation group minus buoyancy plus magnetic damping equals zero.
- S6.1essenceThe sum of A(f′+(η/2)f″) and f′²−ff″. [1.1]
- S6.2essence(A(f′+(η/2)f″) + f′²−ff″) reduced by ((φ_μ + ε)/φ_ρ) f‴ − ((εδ/φ_ρ))f″²f‴. [1.1]
- S6.3essence((A(f′+(η/2)f″) + f′²−ff″) − ((φ_μ + ε)/φ_ρ) f‴ − ((εδ/φ_ρ))f″²f‴) reduced by (φ_β/φ_ρ) λ θ cos α. [1.1]
- S6.4essenceThe sum of (((A(f′+(η/2)f″) + f′²−ff″) − ((φ_μ + ε)/φ_ρ) f‴ − ((εδ/φ_ρ))f″²f‴) − (φ_β/φ_ρ) λ θ cos α) and (φ_σ/φ_ρ) M f′. [1.1]
- S6.5essence((((A(f′+(η/2)f″) + f′²−ff″) − ((φ_μ + ε)/φ_ρ) f‴ − ((εδ/φ_ρ))f″²f‴) − (φ_β/φ_ρ) λ θ cos α) + (φ_σ/φ_ρ) M f′) set equal to 0. [1.1]
- S6The reduced momentum ODE in named dimensionless parameters: the unsteadiness term plus the dimensionless convective inertia minus the Powell–Eyring diffusion/retardation group minus buoyancy plus magnetic damping equals zero.
- Attributes
- S7Balances the Powell–Eyring diffusion and nonlinear retardation against convective inertia, unsteadiness, magnetic damping, and buoyancy; the equation is composed on demand. [1.1]
Dimension II
In practice
How to deal with it
- Consequences
- S8The buoyancy term carries θ, coupling it to the energy ODE.